Use No-Vary-Search for externally conditionalized requests

The implementation of No-Vary-Search support for the net::HttpCache
originally did not intend to support externally conditionalized
requests, as it was not considered useful. However, because `(mode_ &
READ)` is equivalent to `(mode_ & READ_META) || (mode_ & READ_DATA)', in
practice it accidentally supported them. However, an assertion before
sending the request caused it to crash in this case.

It is better for consistency to support No-Vary-Search when JavaScript
gives us a conditional request, and it may be useful for performance as
well, so change to explicitly support No-Very-Search in this case.

This means that requests that have "If-Modified-Since" or
"If-None-Match" headers already on entry to the cache will intentionally
have their URLs rewritten by the NoVarySearchCache if a matching disk
cache entry exists.

BUG=399278577,382394774

Low-Coverage-Reason: TESTS_IN_SEPARATE_CL Tests are coming.
Change-Id: I8b22f81d9aa0ccb384fe70fdcb8b793be4c8838e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6305742
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1426980}